Using GHCi GHCi interpreterGHCi interactiveGHCi GHCi The ‘i’ stands for “Interactive” is GHC's interactive environment, in which Haskell expressions can be interactively evaluated and programs can be interpreted. If you're famililar with HugsHugs , then you'll be right at home with GHCi. However, GHCi also has support for interactively loading compiled code, as well as supporting allexcept the FFI, at the moment the language extensions that GHC provides. FFIGHCi support Foreign Function InterfaceGHCi support Introduction to GHCi Let's start with an example GHCi session. You can fire up GHCi with the command ghci: $ ghci ___ ___ _ / _ \ /\ /\/ __(_) / /_\// /_/ / / | | GHC Interactive, version 5.00, For Haskell 98. / /_\\/ __ / /___| | \____/\/ /_/\____/|_| Type :? for help. Loading package std ... linking ... done. Prelude> There may be a short pause while GHCi loads the prelude and standard libraries, after which the prompt is shown. If we follow the instructions and type :? for help, we get: Commands available from the prompt: <stmt> evaluate/run <stmt> :cd <dir> change directory to <dir> :def <cmd> <expr> define a macro :<cmd> :help, :? display this list of commands :load <filename> load a module (and it dependents) :module <mod> set the context for expression evaluation to <mod> :reload reload the current module set :set <option> ... set options :type <expr> show the type of <expr> :unset <option> ... unset options :quit exit GHCi :!<command> run the shell command <command> Options for `:set' and `:unset': +r revert top-level expressions after each evaluation +s print timing/memory stats after each evaluation +t print type after evaluation -<flag> most GHC command line flags can also be set here (eg. -v2, -fglasgow-exts, etc.) We'll explain most of these commands as we go along. For Hugs users: many things work the same as in Hugs, so you should be able to get going straight away. Haskell expressions can be typed at the prompt: promptGHCi Prelude> 1+2 3 Prelude> let x = 42 in x / 9 4.666666666666667 Prelude> GHCi interprets the whole line as an expression to evaluate. The expression may not span several lines - as soon as you press enter, GHCi will attempt to evaluate it. Loading source files Suppose we have the following Haskell source code, which we place in a file Main.hs in the current directory: main = print (fac 20) fac 0 = 1 fac n = n * fac (n-1) To load a Haskell source file into GHCi, use the :load command: :load Prelude> :load Main Compiling Main ( Main.hs, interpreted ) Ok, modules loaded: Main. Main> GHCi has loaded the Main module, and the prompt has changed to “Main>” to indicate that the current context for expressions typed at the prompt is the Main module we just loaded. So we can now type expressions involving the functions from Main.hs: Main> fac 17 355687428096000 Loading a multi-module program is just as straightforward; just give the name of the “topmost” module to the :load command (hint: :load can be abbreviated to :l). The topmost module will normally be Main, but it doesn't have to be. GHCi will discover which modules are required, directly or indirectly, by the topmost module, and load them all in dependency order. Modules vs. filenames modulesand filenames filenamesof modules Question: How does GHC find the filename which contains module M? Answer: it looks for the file M.hs, or M.lhs. This means that for most modules, the module name must match the filename. If it doesn't, GHCi won't be able to find it. There is one exception to this general rule: when you load a program with :load, or specify it when you invoke ghci, you can give a filename rather than a module name. This filename is loaded if it exists, and it may contain any module you like. This is particularly convenient if you have several Main modules in the same directory and you can't call them all Main.hs. One final note: if you load a module called Main, it must contain a main function, just like in GHC. Making changes and recompilation :reload If you make some changes to the source code and want GHCi to recompile the program, give the :reload command. The program will be recompiled as necessary, with GHCi doing its best to avoid actually recompiling modules if their external dependencies haven't changed. This is the same mechanism we use to avoid re-compiling modules in the batch compilation setting (see ). Loading compiled code compiled codein GHCi When you load a Haskell source module into GHCi, it is normally converted to byte-code and run using the interpreter. However, interpreted code can also run alongside compiled code in GHCi; indeed, normally when GHCi starts, it loads up a compiled copy of package std, which contains the Prelude and standard libraries. Why should we want to run compiled code? Well, compiled code is roughly 10x faster than interpreted code, but takes about 2x longer to produce (perhaps longer if optimisation is on). So it pays to compile the parts of a program that aren't changing very often, and use the interpreter for the code being actively developed. When loading up source files with :load, GHCi looks for any corresponding compiled object files, and will use one in preference to interpreting the source if possible. For example, suppose we have a 4-module program consisting of modules A, B, C, and D. Modules B and C both import D only, and A imports both B & C: A / \ B C \ / D We can compile D, then load the whole program, like this: Prelude> :! ghc -c D.hs Prelude> :load A Skipping D ( D.hs, D.o ) Compiling C ( C.hs, interpreted ) Compiling B ( B.hs, interpreted ) Compiling A ( A.hs, interpreted ) Ok, modules loaded: A, B, C, D. Main> In the messages from the compiler, we see that it skipped D, and used the object file D.o. The message Skipping module indicates that compilation for module isn't necessary, because the source and everything it depends on is unchanged since the last compilation. If we now modify the source of D (or pretend to: using Unix command touch on the source file is handy for this), the compiler will no longer be able to use the object file, because it might be out of date: Main> :! touch D.hs Main> :reload Compiling D ( D.hs, interpreted ) Skipping C ( C.hs, interpreted ) Skipping B ( B.hs, interpreted ) Skipping A ( A.hs, interpreted ) Ok, modules loaded: A, B, C, D. Main> Note that module D was compiled, but in this instance because its source hadn't really changed, its interface remained the same, and the recompilation checker determined that A, B and C didn't need to be recompiled. So let's try compiling one of the other modules: Main> :! ghc -c C.hs Main> :load A Compiling D ( D.hs, interpreted ) Compiling C ( C.hs, interpreted ) Compiling B ( B.hs, interpreted ) Compiling A ( A.hs, interpreted ) Ok, modules loaded: A, B, C, D. We didn't get the compiled version of C! What happened? Well, in GHCi a compiled module may only depend on other compiled modules, and in this case C depends on D, which doesn't have an object file, so GHCi also rejected C's object file. Ok, so let's also compile D: Main> :! ghc -c D.hs Main> :reload Ok, modules loaded: A, B, C, D. Nothing happened! Here's another lesson: newly compiled modules aren't picked up by :reload, only :load: Main> :load A Skipping D ( D.hs, D.o ) Skipping C ( C.hs, C.o ) Compiling B ( B.hs, interpreted ) Compiling A ( A.hs, interpreted ) Ok, modules loaded: A, B, C, D. HINT: since GHCi will only use a compiled object file if it can sure that the compiled version is up-to-date, a good technique when working on a large program is to occasionally run ghc --make to compile the whole project (say before you go for lunch :-), then continue working in the interpreter. As you modify code, the new modules will be interpreted, but the rest of the project will remain compiled. Interactive evaluation at the prompt When you type an expression at the prompt, GHCi immediately evaluates and prints the result. But that's not the whole story: if you type something of type IO a for some a, then GHCi executes it as an IO-computation, and doesn't attempt to print the result:. Prelude> "hello" "hello" Prelude> putStrLn "hello" hello What actually happens is that GHCi typechecks the expression, and if it doesn't have an IO type, then it transforms it as follows: an expression e turns into let it = e; print it which is then run as an IO-action. Hence, the original expression must have a type which is an instance of the Show class, or GHCi will complain: Prelude> id No instance for `Show (a -> a)' arising from use of `print' in a `do' expression pattern binding: print it The error message contains some clues as to the transformation happening internally. What's really in scope at the prompt? When you type an expression at the prompt, what identifiers and types are in scope? GHCi has a concept of a context module, which can be set using the :module command. The context module is shown in the prompt: for example, the prompt Prelude> indicates that the current context for evaluating expressions is the Haskell Prelude module. The Prelude is the default context when you start up GHCi. Prelude Exactly which entities are in scope in a given context depends on whether the context module is compiled or interpreted: If the context module is interpreted, then everything that was in scope during compilation of that module is also in scope at the prompt, i.e. all the imports and any top-level functions, types and classes defined in that module. If the context module comes from a package, or is otherwise compiled, then only the exports of that module are in scope at the prompt. So for example, when the current context module is Prelude, everything the Prelude exports is in scope, but if we switch context to eg. Time, then everything from the Prelude is now invisible. The reason for this unfortunate distinction is boring: for a compiled module when the source isn't available, the compiler has no way of knowing what was in scope when the module was compiled (and we don't store this information in the interface file). However, in practice it shouldn't be a problem: if you want both Time and Prelude in scope at the same time, just create a file containing the line import Time and load it into GHCi. To make life slightly easier, the GHCi prompt also behaves as if there is an implicit import qualified declaration for every module in every package, and every module currently loaded into GHCi. So in the above example where the Prelude was invisible, we can always get at Prelude identifiers by qualifying them, eg. Using <literal>do-</literal>notation at the prompt do-notationin GHCi statementsin GHCi GHCi actually accepts statements rather than just expressions at the prompt. This means you can bind values and functions to names, and use them in future expressions or statements. The syntax of a statement accepted at the GHCi prompt is exactly the same as the syntax of a statement in a Haskell do expression. However, there's no monad overloading here: statements typed at the prompt must be in the IO monad. Here's an example: Prelude> x <- return 42 Prelude> print x 42 Prelude> The statement x <- return 42 means “execute return 42 in the IO monad, and bind the result to x”. We can then use x in future statements, for example to print it as we did above. Of course, you can also bind normal non-IO expressions using the let-statement: Prelude> let x = 42 Prelude> print x 42 Prelude> An important difference between the two types of binding is that the monadic bind (p <- e) is strict (it evaluates e), whereas with the let form, the expression isn't evaluated immediately: Prelude> let x = error "help!" Prelude> print x *** Exception: help! Prelude> Any exceptions raised during the evaluation or execution of the statement are caught and printed by the GHCi command line interface (see for more information on GHC's Exception support). Every new binding shadows any existing bindings of the same name, including entities that are in scope in the current module context. WARNING: temporary bindings introduced at the prompt only last until the next :load or :reload command, at which time they will be simply lost. However, they do survive a change of context with :module: the temporary bindings just move to the new location. HINT: if you turn on the +t option, GHCi will show the type of each variable bound by a statement. For example: +t Prelude> :set +t Prelude> let (x:xs) = [1..] x :: Integer xs :: [Integer] The <literal>it</literal> variable it Whenever an expression (or a non-binding statement, to be precise) is typed at the prompt, GHCi implicitly binds its value to the variable it. For example: Prelude> 1+2 3 Prelude> it * 2 6 This is a result of the translation mentioned earlier, namely that an expression e is translated to let it = e; print it before execution, resulting in a binding for it. If the expression was of type IO a for some a, then it will be bound to the result of the IO computation, which is of type a. eg.: Prelude> Time.getClockTime Prelude> print it Wed Mar 14 12:23:13 GMT 2001 The corresponding translation for an IO-typed e is it <- e Note that it is shadowed by the new value each time you evaluate a new expression, and the old value of it is lost. Invoking GHCi invokingGHCi GHCi is invoked with the command ghci or ghc --interactive. A module or filename can also be specified on the command line; this instructs GHCi to load the that module or filename (and all the modules it depends on), just as if you had said :load module at the GHCi prompt (see ). For example, to start GHCi and load the program whose topmost module is in the file Main.hs, we could say: $ ghci Main.hs Note: only one module name or filename may be given on the command line. Most of the command-line options accepted by GHC (see ) also make sense in interactive mode. The ones that don't make sense are mostly obvious; for example, GHCi doesn't generate interface files, so options related to interface file generation won't have any effect. Packages packageswith GHCi GHCi can make use of all the packages that come with GHC, For example, to start up GHCi with the text package loaded: $ ghci -package text ___ ___ _ / _ \ /\ /\/ __(_) / /_\// /_/ / / | | GHC Interactive, version 5.00, For Haskell 98. / /_\\/ __ / /___| | \____/\/ /_/\____/|_| Type :? for help. Loading package std ... linking ... done. Loading package lang ... linking ... done. Loading package text ... linking ... done. Prelude> Note that GHCi also loaded the lang package even though we didn't ask for it: that's because the text package makes use of one or more of the modules in lang, and therefore has a dependency on it. The following command works to load new packages into a running GHCi: Prelude> :set -package name But note that doing this will cause all currently loaded modules to be unloaded, and you'll be dumped back into the Prelude. Extra libraries librarieswith GHCi Extra libraries may be specified on the command line using the normal -llib option. For example, to load the “m” library: $ ghci -lm On systems with .so-style shared libraries, the actual library loaded will the GHCi searches the following places for libraries, in this order: Paths specified using the -Lpath command-line option, the standard library search path for your system, which on some systems may be overriden by setting the LD_LIBRARY_PATH environment variable. On systems with .dll-style shared libraries, the actual library loaded will be lib.dll. Again, GHCi will signal an error if it can't find the library. GHCi can also load plain object files (.o or .obj depending on your platform) from the command-line. Just add the name the object file to the command line. GHCi commands GHCi commands all begin with ‘:’ and consist of a single command name followed by zero or more parameters. The command name may be abbreviated, as long as the abbreviation is not ambiguous. All of the builtin commands, with the exception of :unset and :undef, may be abbreviated to a single letter. :cd dir :cd Changes the current working directory to dir. A ‘˜’ symbol at the beginning of dir will be replaced by the contents of the environment variable HOME. :def name expr :def The command :def name expr defines a new GHCi command :name, implemented by the Haskell expression expr, which must have type String -> IO String. When :name args is typed at the prompt, GHCi will run the expression (name args), take the resulting String, and feed it back into GHCi as a new sequence of commands. Separate commands in the result must be separated by ‘\n’. That's all a little confusing, so here's a few examples. To start with, here's a new GHCi command which doesn't take any arguments or produce any results, it just outputs the current date & time: Prelude> let date _ = Time.getClockTime >>= print >> return "" Prelude> :def date date Prelude> :date Fri Mar 23 15:16:40 GMT 2001 Here's an example of a command that takes an argument. It's a re-implementation of :cd: Prelude> let mycd d = Directory.setCurrentDirectory d >> return "" Prelude> :def mycd mycd Prelude> :mycd .. Or I could define a simple way to invoke “ghc --make Main” in the current directory: Prelude> :def make (\_ -> return ":! ghc --make Main") :help :help :? :? Displays a list of the available commands. :load module :load Recursively loads module, and all the modules it depends on. Here, module must be a module name or filename, but may not be the name of a module in a package. All previously loaded modules, except package modules, are forgotten. The module module is known as the target. After a :load command, the current context is set to: module, if it was loaded successfully, or the most recently successfully loaded module, if any other modules were loaded as a result of the current :load, or Prelude otherwise. :module module :module Sets the current context for statements typed at the prompt to module, which must be a module name which is already loaded or in a package. See for more information on what effect the context has on what entities are in scope at the prompt. :quit :quit Quits GHCi. You can also quit by typing a control-D at the prompt. :reload :reload Attempts to reload the current target (see :load) if it, or any module it depends on, has changed. Note that this may entail loading new modules, or even dropping modules which are no longer indirectly required by the target. :set option... :set Sets various options. See for a list of available options. The :set command by itself shows which options are currently set. :type expression :type Infers and prints the type of expression, including explicit forall quantifiers for polymorphic types. The monomorphism restriction is not applied to the expression during type inference. :undef name :undef Undefines the user-defined command name (see :def above). :unset option... :unset Unsets certain options. See for a list of available options. :! command... :! shell commandsin GHCi Executes the shell command command. The <literal>:set</literal> command :set The :set command sets two types of options: GHCi options, which begin with ‘+” and “command-line” options, which begin with ‘-’. GHCi options optionsGHCi GHCi options may be set using :set and unset using :unset. The available GHCi options are: +r +r CAFsin GHCi Constant Applicative FormCAFs Normally, any evaluation of top-level expressions (otherwise known as CAFs or Constant Applicative Forms) in loaded modules is retained between evaluations. Turning on +r causes all evaluation of top-level expressions to be discarded after each evaluation (they are still retained during a single evaluation). This option may help if the evaluated top-level expressions are consuming large amounts of space, or if you need repeatable performance measurements. +s +s Display some stats after evaluating each expression, including the elapsed time and number of bytes allocated. NOTE: the allocation figure is only accurate to the size of the storage manager's allocation area, because it is calculated at every GC. Hence, you might see values of zero if no GC has occurred. +t +t Display the type of each variable bound after a statement is entered at the prompt. If the statement is a single expression, then the only variable binding will be for the variable ‘it’. Setting GHC command-line options in GHCi Normal GHC command-line options may also be set using :set. For example, to turn on , you would say: Prelude> :set -fglasgow-exts Any GHC command-line option that is designated as dynamic (see the table in ), may be set using :set. To unset an option, you can set the reverse option: dynamicoptions Prelude> :set -fno-glasgow-exts lists the reverse for each option where applicable. Certain static options (, , , and in particular) will also work, but some may not take effect until the next reload. staticoptions The <filename>.ghci</filename> file .ghcifile startupfiles, GHCi When it starts, GHCi always reads and executes commands from $HOME/.ghci, followed by ./.ghci. The .ghci in your home directory is most useful for turning on favourite options (eg. :set +s), and defining useful macros. Placing a .ghci file in a directory with a Haskell project is a useful way to set certain project-wide options so you don't have to type them everytime you start GHCi: eg. if your project uses GHC extensions and CPP, and has source files in three subdirectories A B and C, you might put the following lines in .ghci: :set -fglasgow-exts -cpp :set -iA:B:C (Note that strictly speaking the flag is a static one, but in fact it works to set it using :set like this. The changes won't take effect until the next :load, though.) FAQ and Things To Watch Out For GHCi complains about main not being in scope when I load a module. mainwith GHCi You probably omitted the module declaration at the top of the module, which causes the module name to default to Main. In Haskell, the Main module must define a function called main. Admittedly this doesn't make a great deal of sense for an interpreter, but the rule was kept for compatibility with GHC. System.exit causes GHCi to exit! System.exitin GHCi Yes, it does. System.getArgs returns GHCi's command line arguments! Yes, it does. The interpreter can't load modules with FFI declarations! Unfortunately not. We haven't implemented it yet. Please compile any offending modules by hand before loading them into GHCi. Hugs has a :add command for adding modules without throwing away any that are already loaded. Why doesn't this work in GHCi? We haven't implemented it yet. Sorry about that. -O doesn't work with GHCi! For technical reasons, the bytecode compiler doesn't interact well with one of the optimisation passes, so we have disabled optimisation when using the interpreter. This isn't a great loss: you'll get a much bigger win by compiling the bits of your code that need to go fast, rather than interpreting them with optimisation turned on. Unboxed tuples don't work with GHCi That's right. You can always compile a module that uses unboxed tuples and load it into GHCi, however. (Incidentally the previous point, namely that -O is incompatible with GHCi, is because the bytecode compiler can't deal with unboxed tuples). Concurrent threads don't carry on running when GHCi is waiting for input. No, they don't. This is because the Haskell binding to the GNU readline library doesn't support reading from the terminal in a non-blocking way, which is required to work properly with GHC's concurrency model.